Investigation and comparison of generators for dynamic operation in ocean buoys

Abstract:

A significantly untapped renewable energy source exists in the world's oceans. It is estimated that if 0.2% of the ocean's untapped energy could be harnessed, it could
provide power sufficient for the entire world. Ocean energy extraction is an old concept, and it is currently seeing the benefit of advancing support technologies and an increasing awareness of potential future energy deficits. Girard, in Paris, registered the world's first wave energy device patent in 1799. Over more than 200 years of development, the number of patents has greatly increased and today there are currently more than 1500 ocean wave energy device patents. This thesis presents and examines the concept of using an ocean wave as the prime mover for a buoy-mounted, directly driven, electrical generator. The primary focus will be on two particular generator types, a Permanent Magnet Synchronous Generator (PMSG) and a Wound Rotor Induction Generator (WRIG), and their reactions to a simulated wave profile. The simulations were based on the concept of a directly driven screw, either a roller screw or ball screw, and a sinusoidal wave profile.
The development of this thesis moves through four sections: introduction of wave energy, the details of the experimentation that was conducted; the results of those experiments and the comparisons with mathematical models; and finally the conclusion and recommendations for future experimentation in this area. A number of different experiments have been conducted for this thesis on the
directly driven buoy generator scenario. They range from baseline control tests, which
establish a characterization of the generators, to intricate multi-directional
reciprocating drive research. The reciprocating drive scenario is used to simulate a unidirectional direct drive,
where one half of the stroke is used to generate electrical power and the energy from
the other half of the stroke is stored in inertial mass, such as a spring, or some other similar energy storage device. The experiments were conducted on both the PMSG and the WRIG as was appropriate to their characteristics, to see how each of the platforms reacts to similar prime movers. Experimental data from the PMSG and WRIG investigations will help to home in on which type of generator would be more suitable for a given buoy generator design. The designation will be based on two main factors, the physical generator to drive screw connection, and the power take-off (PTO) system to be implemented between the buoy generators and the electrical utility grid. Additionally, an over-running clutch connection will be implemented to verify the
notion of a partial cycle "free-wheeling" generation scheme based on the inertial force
of the generator itself. It has been hypothesized that if the inertial force of the generator were greater than the load force from the output connection, then the generator would be allowed to "free-wheel" through half of the wave cycle. This will
allow the generator to remain rotating in one direction, as opposed to a directly coupled connection which would force the direction of rotation to oscillate from clockwise to counterclockwise as the buoy driving the generator floated over a wave front. By creating a unidirectional rotation, the ability to use a WRIG becomes much more applicable due to the requirement for fairly high rotational speed for generation. Much like a child's mechanical top, a WRIG will be able to run up to a base speed suitable for generation and then the electrical utility grid connection will be energized. By contrast, the PMSG would likely be used in a reciprocating scenario thereby taking advantage of the intrinsic constant magnetic field, which will produce a usable output at low rotational speeds. It is concluded that each of these generators will have a potential niche to fill in Ocean Energy extraction buoys. Due to the reciprocating nature of the ocean waves and the varying periods and magnitudes, some form of the output conditioning will need to be applied if there are plans for a direct grid connection. The WRIG outputs a fairly constant voltage but the current magnitude varies with the wave profile. If some form of current leveling was introduced and the output current magnitude was made more constant the WRIG will be the prime candidate for a direct grid connection. The PMSG, and its ease of output rectification make it the choice for a rectified transmission to shore scheme, where it can then be inverted to undistorted ac power and connected into the electrical utility grid.